To generate an image using a medical ultrasound imaging system, such as an intravascular ultrasound (IVUS) or intracardiac echocardiography (ICE) imaging system, an ultrasound imaging device, typically including one or more transducers, is located on or within an intravascular device, such as a catheter and the like. The intravascular device is navigated into the body and the imaging device is used to image the desired body tissue. To do this, the transducer generates and transmits an ultrasound pulse into the body tissue. As this pulse strikes various layers of body tissue, echoes are reflected back to and received by the transducer. The transducer generates an electrical output signal representative of the strength of the received echo and outputs this signal to an image processing system. The image processing system processes the signal and uses it to form an image of the body tissue.
Conventionally, ultrasound transducers have been made of piezoelectric materials which require ceramic manufacturing technologies which are vastly different from those used to manufacture other components in an ultrasound imaging system, which are typically semiconductor-based. Piezoelectric transducers typically have a narrow bandwidth which limits the depth of tissue that can be imaged.
Recently, a new type of transducer has been developed capable of fabrication with semiconductor-based processing technologies. Capacitive micromachined ultrasonic transducers (CMUTs) were designed to answer a need to mass fabricate medical, ultrasound transducers using the very same semiconductor manufacturing processes used to fabricate the other parts of an external ultrasound imaging system. CMUTs are typically much smaller than piezoelectric transducers (on the order of 10 to 100 microns in size) and have a larger bandwidth.
A typical CMUT includes a drumhead structure suspended over a substrate in a manner to allow two-way conversion between a mechanical wave and an electrical signal through modulation of a capacitive charge on the drumhead. To deliver an ultrasound pulse, the capacitive charge on the drumhead, measured relative to a substrate electrode, is modulated by delivery of an electrical pulse to the drumhead. The delivery of this pulse causes the drumhead to vibrate and thereby transmit an ultrasound wave. Likewise, in the receiving mode, the impact of the echo on the drumhead modulates the capacitance and results in an electrical signal representative of the strength of the received echo.
CMUT devices are not currently used in IVUS imaging systems: One reason for this is because in order to operate, the CMUT needs a constant bias voltage that is carefully controlled so as to maintain high transducer sensitivity without short-circuiting the transducer's capacitance. Accordingly, there is a need for systems and methods for applying this bias voltage to CMUT devices in intravascular ultrasound imaging systems.